Membrane permeate recycle process for use with pressure swing adsorption processes

ABSTRACT

Process of treating a net gas stream is disclosed. The process includes sending the net gas stream to a compressor to produce a compressed gas stream. The compressed gas stream is then sent to a pressure swing adsorption unit to produce a hydrogen product stream and a tail gas stream. Tail gas stream from the pressure swing adsorption unit is sent to a first membrane unit to produce a first permeate stream and a first non-permeate stream. Portion of the tail gas stream is sent to a second membrane unit to produce a second permeate stream and a second non-permeate stream.

FIELD OF THE INVENTION

The present disclosure relates to a membrane permeate recycle processutilizing hydrogen recovery units and pressure swing adsorption units.

BACKGROUND OF THE INVENTION

Typically, a membrane separation process includes a feed conditioningsection that conditions the membrane feed gas by removing liquids,solids and undesired contaminants and then establishes the desiredmembrane operating temperature. The separation of gases in the membraneseparation process is achieved due to the difference in relativepermeation rates of hydrogen and other hydrocarbon components when apressure difference between a feed side and a permeate side of asemi-permeable membrane barrier is present or imposed. Typically,membrane elements contain the semi-permeable membrane barrier.

Conventionally, membrane separation units contain at least two membranemodule banks. The membrane module banks of the membrane separationsection are arranged in parallel and connect into common piping thattypically distributes the membrane feed gas and collects the membranepermeate gas and membrane non-permeate gas streams. The knownnon-permeate conditioning section contains an automatic control valvethat maintains the pressure on the membrane side and can containadditional equipment such as heat exchangers and knock out drums to coolthe gas and remove any condensed liquids produced after cooling.

For this reason, at least two membrane module banks are installed ineach membrane unit so that a turndown control of 25-100% of the nominalflow rate can be achieved by isolating the individual membrane modulebanks when feed flow capacity reduces in combination with the stage-cutcontrol on the membrane module banks are in service.

Accordingly, it is desirable that an improved process is provided whichwould be an enhancement of the existing hydrogen recovery flow schemeassociated with a catalytic reformer. A need exists for improvement inmembrane separation process for improving the permeate recycle of thegases thereby reducing the operating and capital expenses of the overallmembrane separation process and unit. Consequently, an improved pressureswing adsorption unit is required that can be used for refinery off-gasseparation where high recovery of hydrogen and LPG is desirable and thedownstream consumers are using the product hydrogen at high pressure inprocessing reactors such as hydroprocessing. Further, there is requireda separation process which is dynamic and robust in operation.

Furthermore, other desirable features and characteristics of the presentsubject matter will become apparent from the subsequent detaileddescription of the subject matter and the claims, taken in conjunctionwith the accompanying drawing and this background of the subject matter.

SUMMARY OF THE INVENTION

Various embodiments contemplated herein relate to an improved permeaterecycle process which includes a pressure swing adsorption unit and amembrane unit to which a membrane permeate stream is recycled formaximum hydrogen recovery. In accordance with an exemplary embodiment, aprocess is provided for treating a net gas stream comprising sending thenet gas stream to a compressor to produce a compressed gas stream. Thecompressed net gas stream is sent to a pressure swing adsorption unit toproduce a hydrogen product stream and a tail gas stream. The tail gasstream is sent to a first membrane unit to produce a first permeatestream and a first residual stream. A portion of the tail gas stream isfurther sent to a second membrane unit to produce a second permeatestream and a second residual stream.

In accordance with another exemplary embodiment, a process for treatinga net gas stream is provided comprising sending the net gas stream to acompressor for producing a compressed gas stream which is further passto a pressure swing adsorption unit for recovering hydrogen as a productstream along with a tail gas stream. The process further includessending the recovered tail gas stream to a first membrane unit forproducing a first permeate stream and a first non-permeate stream. Thetail gas stream is then sent to a second membrane unit to produce asecond permeate stream and a second non-permeate stream followed bycontrolling the flow of the tail gas stream to the first membrane unitand second membrane unit.

Accordingly, the present disclosure, describes an improved permeaterecycle process that reduces operating and capital expenses includingthe control system requirements to make the system fully flexible andadaptable to changes in system operation. Applicants have found that theinstant solution is achieved by adding a second membrane unit thatoperates at lower pressure as compared to the first membrane unit whichreduces the overall size of the membrane unit required for the instantflow scheme. Also, the recycle of the permeate gas from the secondmembrane unit to the pressure swing adsorption (PSA) unit is kept at lowpressure, thereby lowering the operational and capital cost of therecycle operation. Further, the low pressure permeates recycle from thesecond membrane unit provides purge gas sent to the PSA unit resultingin an increase in the hydrogen recovery from the PSA unit. Accordingly,the current permeates recycle process reduces the operating expenses byabout 10% and reduces capital costs by about 7%.

These and other features, aspects, and advantages of the presentinvention will become better understood upon consideration of thefollowing detailed description, drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The various embodiments will hereinafter be described in conjunctionwith following figures, wherein like numerals denote like elements.

FIG. 1 is a schematic process flow diagram illustrating the prior artmembrane separation process.

FIG. 2 is a schematic flow diagram of the process of the presentdisclosure illustrating the improvement in membrane permeate recycleprocess.

FIG. 3 illustrates a reduction in the power required to run the systemof the present disclosure.

FIG. 4 illustrates the significant reduction in the membrane area asrequired in the present disclosure.

FIG. 5 is a schematic illustrating the details of the piping layout asused in the present disclosure where the two membrane units areintegrated into a single membrane system.

Skilled artisans will appreciate that elements described in FIGS. 1-5are illustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the dimensions of some of the elements inFIGS. 1-5, may be exaggerated relative to other elements to help toimprove understanding of various embodiments of the present disclosure.Also, common but well-understood elements that are useful or necessaryin a commercially feasible embodiment may not be depicted in order tofacilitate a less obstructed view of these various embodiments of thepresent disclosure.

Definitions

As used herein, the term “stream” can include various hydrocarbonmolecules and other substances.

As used herein, the term “rich” can mean an amount of generally at leastabout 50% or at least about 70%, preferably about 90%, and optimallyabout 95%, by mole, of a compound or class of compounds in a stream.

As used herein the term “fluid communication” means that materialflowing in between the enumerated components is in fluid state and itconnects the two components.

As used herein, the term “permeate stream” can mean the product streamallowed to pass through the membranes.

As used herein, the term “non-permeate stream” can mean the retentatestream which is not allowed to pass through the membranes and remains onthe membrane.

As used herein, the term “membrane” can mean a selective barrier, thatallows some things to pass through it or permeate but stops others whichremains as a retentate.

As used herein, the term “C_(x)” wherein “x” is an integer means ahydrocarbon stream with hydrocarbons having x carbon atoms.

As used herein, the term “C_(x−)” wherein “x” is an integer means ahydrocarbon stream with hydrocarbons having x and/or less carbon atomsand preferably x and less carbon atoms.

As used herein, the term “C_(x+)” wherein “x” is an integer means ahydrocarbon stream with hydrocarbons having x and/or more carbon atomsand preferably x and more carbon atoms.

As used herein, the term “stage-cut” can be determined as the ratio ofthe permeate flow rate to the membrane feed gas flow rate at a specifiedvalue.

As used herein, the term “bank” can refer to a set of each parallelmembrane modules that can be fully isolated from the rest of theprocess.

As used herein, the term “separator” means a vessel which has an inletand at least an overhead vapor outlet and a bottoms liquid outlet andmay also have an aqueous stream outlet from a boot.

As used herein, the term “portion” means an amount or part taken orseparated from a main stream without any change in the composition ascompared to the main stream. Further, it also includes splitting thetaken or separated portion into multiple portions where each portionretains the same composition as compared to the main stream.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the various embodiments or the application anduses thereof. Furthermore, there is no intention to be bound by anytheory presented in the preceding background or the following detaileddescription. The Figures have been simplified by the deletion of manyapparatuses customarily employed in a process of this nature, such asvessel internals, temperature and pressure controls systems, flowcontrol valves, recycle pumps, etc. which are not specifically requiredto illustrate the performance of the invention. Furthermore, theillustration of the process of this invention in the embodiment of aspecific drawing is not intended to limit the invention to specificembodiments set out herein.

As depicted, process flow lines in the figures can be referred to,interchangeably, as, e.g., lines, pipes, branches, distributors,streams, effluents, feeds, products, portions, catalysts, withdrawals,recycles, suctions, discharges, and caustics.

The instant disclosure provides an efficient way to recycle hydrogenwithin the process that is accomplished by operating part of thetail-gas membrane cartridges at lower permeate pressure, and using thispermeate as external purge gas in the PSA unit. This results in asignificant reduction in total installed membrane area and a significantreduction in compression power.

An embodiment of membrane permeates recycle process is addressed andshown in FIG. 1. The flow scheme of FIG. 1 shows a net gas compressionand hydrocarbon recovery system 20 to which a net gas stream 10containing mainly hydrogen rich gas is being passed. The liquidsproduced after the compression and recovery are collected as heavyhydrocarbon stream in line 24 and the compressed gas in line 22containing hydrogen and light hydrocarbons is passed to a high-pressurePSA unit 30. The PSA unit 30 delivers the hydrogen rich gas stream inline 32 to a first stage compressor 40 for further compression and thehydrogen rich gas stream is recovered in line 42. The first stagecompressor 40 compresses the hydrogen rich gas stream to a pressuretypically from 250-400 psia to 600-900 psia.

The first stage compressor unit 40 delivers the recovered hydrogen 42 athigh purity (>99.9 mol %) which is required to be further compressed.The non-recovered hydrogen and hydrocarbon impurities are recovered asPSA tail gas in line 34 that is compressed in a PSA tail gas compressor50 with a compressed tail gas stream in line 52 sent to a first membraneunit 60 to produce a first permeate stream in line 62 which is sent to adryer 70. The dried gas in line 72 is recycled and mixed with the netgas feed stream in line 10. The dryer 70 may include adsorbents toremove moisture (H2O) from PSA tail gas stream in line 62.

The first non-permeate stream in line 64 from the first membrane unit 60can be combined with other streams from a catalytic reforming unit.Reactor effluent in line 110 (from reactor, not shown) is injected intoa separator unit 80 to produce a separator liquid in line 82. Theseparator effluent stream in line 82 is mixed with a hydroprocessingstripper off-gas stream (supplied from an external source) via line 120and the first non-permeate stream in line 64 forming a mixed effluentstream in line 84. Further, the mixed effluent stream in line 84 is thenfully supplied to an absorber unit 90 installed downstream to the firstmembrane unit. The absorber 90 produces a fuel gas stream taken from thetop of the absorber in line 92 and a heavy hydrocarbon stream in thebottom line 94. The heavies stream in line 94 can be further treated ina depentanizer or a debutanizer of the catalytic reforming unit, toseparate out pentanes and butanes, respectively.

The heavy hydrocarbon bottom stream can further be combined with anotherheavier hydrocarbon stream supplied in line 24 and recovered as aneffluent from the net gas compressor and hydrocarbon recovery system 20forming a mixed heavy hydrocarbon product stream in line 96. The reactoreffluent stream in line 110 is recovered as a catalytic reformingeffluent stream comprising hydrogen, light hydrocarbons (from C1 to C4),light naphtha (C5 to C6) and heavy naphtha (C6 to C11) range materials.Accordingly, reformate effluent stream in line 110 may be passed to theseparator 80. In the separator 80, vapors may be separated to provide areformate vapor stream (not shown) and a reformate liquid stream in line82.

Referring now to FIG. 2, an embodiment for improved and efficienthydrogen recovery from the membrane permeates recycle process is shown.The flow scheme as shown in FIG. 2, has a benefit provided in that apart of the pressure swing adsorption (PSA) tail gas is recycled at lowpressure, thereby avoiding costly re-compression and reducing the sizeof the recycle compressor. FIG. 2 shows the flow scheme of the presentinvention with a net gas stream 10 containing mainly hydrogen rich gas(major hydrogen and remaining hydrocarbons) from a catalytic reformingunit sent to a net gas compressor and hydrocarbon recovery system 20.The liquids produced after the compression and recovery are collected asheavy hydrocarbon stream in line 24. The compressed gas in line 22containing hydrogen and light hydrocarbons, then passes to a first stagecompressor 30 with a further compressed stream in line 32 sent to ahigh-pressure PSA unit 40.

However, there could be an alternative flow scheme (not shown here) forutilizing a PSA unit 40 operating at lower pressure which would directlypass the compressed gas in line 22 into the PSA unit 40 without the needfor first stage compressor 30. Further, the compression system 30compresses the gas in line 22 to a pressure ranging typically from250-400 psia to 600-900 psia. The PSA unit 40 delivers the recoveredhydrogen 42 at high purity (>99.9 mol %) to the consumer. Thenon-recovered hydrogen and hydrocarbon impurities collectively makes upthe PSA tail gas stream in line 44 that is further compressed byutilizing a PSA tail gas compressor 50 and recovering a compressed tailgas stream in line 52 which is further passed on to a dryer unit 60. Thecompressor 50 compresses the PSA tail gas in line 44 at a pressureranging typically from 15-25 psia to 250-350 psia.

A first portion of the dried gas in line 62 is taken in line 66 anddirected to a first membrane unit 70 to produce a first permeate streamin line 72 which is recycled to and mixed with the net gas feed streaminjected in line 10. A second portion of the dried gas in line 62 istaken in line 64 which is sent as a feed gas to a second membrane unit80 to produce a second permeate stream in line 82 being recycled to thePSA unit 40. In an aspect of the present disclosure, the dryer unit 60may also include a separator filled with adsorbents to remove moisture(H2O) from the PSA tail gas stream.

The first non-permeate stream recovered as an effluent in line 74 fromthe first membrane unit 70 and the second non-permeate stream recoveredas effluent in line 84 from the second membrane unit 80 are combined andmixed with other streams from the catalytic reforming unit (not shownhere). The reactor effluent coming in, in line 110 is injected into aseparator unit 90 thereby, producing a separator liquid stream in line92. The separator liquid stream 92 is further mixed with hydroprocessingstripper off-gas stream injected via external source in line 120 andcombined with the first non-permeate stream 74 and the secondnon-permeate streams in line 84 to form a combined effluent stream 94which goes to the absorber unit 100. The absorber unit 100 produces afuel gas stream recovered at the top of the absorber from line 102 and aheavier hydrocarbon stream recovered from bottom of the absorber vialine 104. The heavy hydrocarbon bottom stream 104 can be further treatedin a depentanizer or a debutanizer unit of a catalytic reforming unit.Also, the heavy hydrocarbon bottom stream recovered in line 104 can bemixed with the heavier hydrocarbon stream flowing in line 24 andrecovered as effluent from the net gas compressor and hydrocarbonrecovery system 20. The mixed resultant stream flows in line 106. Thereactor effluent stream in line 110 is recovered as a catalyticreforming effluent stream comprising hydrogen, light hydrocarbons (fromC₁ to C₄), light naphtha (C₅ to C₆) and heavy naphtha (C₆ to C₁₁) rangematerials.

As an alternative feature either a portion of or all of the fuel gasstream in line 102 can also be recycled to the net gas compressor andhydrocarbon recovery system 20 to recover hydrogen out of the fuel gasstream (C²⁻). In an aspect of the present invention, the net gascompression and hydrocarbon recovery system 20 may include a separatorin fluid communication with the compressor to separate any liquidpresent and to pass the vapor or gas portion of the stream to the nextprocess step or compression step. Further, coolers may also be presenttherein for cooling. Further, the compressor may have a maximum of twostages. The membrane feed pre-treatment section is commonly foundcomprising of a feed dryer, feed filter or a coalescer or a feedknock-out drum and feed flow measurement with pressure and temperaturecompensation. A feed heater is installed in the membrane pre-treatmentsection to condition the membrane feed gas temperature at a constantvalue.

Also, the separation of gases in the membrane separation unit takesplace due to differences in relative permeation rates of hydrogen andother hydrocarbon components when a pressure difference between feedside and permeate side of a semi-permeable membrane barrier is imposed.The semi-permeable membrane barrier is contained within the membraneelements. This semi-permeable membrane barrier performs the separationand is typically, though not limited to, formed of a material selectedfrom the group of cellulose acetate, polyimide or polysulphone, etc.,showing a selectivity between permeable molecules like hydrogen and lesspermeable molecules such as hydrocarbons. In the improved flow scheme,as shown in FIG. 2, the part of the PSA tail gas is sent to anothermembrane unit rather than the single membrane unit that is used inschematics shown in FIG. 1. This membrane unit operates with a lowerpermeate flow draw-off and lower permeate pressure (20 psig) compared tothe main tail-gas membrane (85 psig permeate pressure). Because of thelower permeate pressure, the driving force for permeation is larger. Thereduced permeate pressure increases the membrane feed to permeatepressure ratio. Lowering the permeate pressure from 85 psig (100 psia)to 20 psig (35 psia) increases this ratio by a factor of almost 3 andreduces the required membrane area by a factor 3. Because the cost ofmembrane systems is proportional to the installed membrane area, thisresults in a significant cost reduction.

A further reduction in required membrane area is coming from thepermeate flow draw-off. Permeate from the smaller membrane is recycleddirectly to the PSA unit instead of through the PSA feed re-contact andcompression sections and is no longer utilized as PSA feed gas but isused instead as PSA purge gas. Hydrogen purity of this purge gas is high(about 97 mol % or higher), whereas permeate recycle from the maintail-gas membrane has a lower hydrogen purity (of about 67 mol %). Theuse of the recycled hydrogen as purge gas in the PSA unit permits anincreased recovery of hydrogen in the PSA process as the amount of purgegas that is normally used for internal purge can now be used forpressure equalizations during co-current depressurization throughfurther optimization of the PSA cycle equalization steps. The reductionin recycle flow rate slightly improves the PSA feed gas quality whichalso has a beneficial effect on the PSA unit recovery and requiredadsorber bed volume, both of which have a positive impact on the PSAunit cost.

The flow scheme of FIG. 2, with two membrane units can be made veryflexible as the split of PSA tail gas flow rates to each of the twomembranes can be optimized for any given case and is determined byoperating conditions (pressure) of the process and the desired overallhydrogen recovery. In this case at a refinery, 77% of PSA tail gas goesto the smaller membrane that generates the recycle purge gas.

In an exemplary embodiment, the illustrative representation of therelative power requirements by the two systems have been shown. FIG. 3,depicts the relative difference in power required by the prior artsystem and instant disclosure. The readings shown by the upper linerepresents power required for operating the process as depicted in priorart FIG. 1 and the readings shown by the lower line represents the powerrequired to carry out the instant process as per the applicants'disclosure. Further a comparison of the improved scheme with that of theprior art system, is depicted in FIG. 3 over a range of hydrogenrecoveries. Results show a consistent operating benefit of ˜10% lowercompression power. An important aspect of this invention is related tooperating pressure of the PSA unit. It was found that the benefit ofusing an external purge gas in the PSA cycle is greater for higherpressure ratios (feed pressure over purge pressure) in the PSA unit. PSAunits operating at higher pressure can perform additional pressureequalization steps that increase the hydrogen recovery.

Further, as noted in the present invention, a PSA pressure ratio of 37was used to maximize the efficiency of external purge. This PSA pressureratio is determined based on the ratio of feed gas pressure to the tailgas pressure, i.e., 815 (psia)/22 (psia). To achieve this high PSApressure ratio, the PSA feed gas is compressed and a high-pressure cycleis used to generate product hydrogen for downstream high-pressureconsumers (e.g., hydrocracker).

Referring next to FIG. 4, which shows the total membrane area requiredpreviously is shown by the upper line and the total membrane area nowrequired by the present disclosure is shown by the lower line, with thetotal membrane area reduced by 1.4× to 5× times over a range of hydrogenrecovery from 96.5% to 98.5%, respectively.

FIG. 5 shows the piping details of the permeate section of a system thatcombines first membrane unit 70 and second membrane unit 80 into anintegrated membrane unit still meeting the requirements of the flowscheme of FIG. 2 of this invention. Further FIG. 5 shows three membranemodule banks 220, 230 and 240 that each contain individual membranemodules 222, 232, 242. Each membrane module bank can be isolated by bankisolation valves 226 and 228, 236 and 238, and 246 and 248 from themembrane permeate headers. Inside the membrane module banks 220, 230 and240, lines 224, 234 and 244 connect the membrane modules that areinstalled in parallel. Segregation valves 252 and 254 are in openposition, connecting banks 230 and 240 to stage 1. Segregation valves250 and 260 are in closed position, separating bank 220 from the otherbanks 230 and 240 and thereby making up stage 2.

The first membrane unit operating at high permeate pressure is at stage1 and the second membrane unit operating at low permeate pressure is atstage 2. Permeate product 270 from stage 1 and permeate product 268 fromstage 2 are withdrawn at opposite ends of the permeate header. Controlvalve 256 controls the pressure in stage 2 in permeate line 268 whilecontrol valve 258 controls the lower pressure in stage 1 in permeateline 270.

An additional feature of the present invention is the integratedmembrane units and control of the integrated membrane units into asingle unit having two membrane separation sections operating atdifferent permeate pressures. The integrated membrane unit of thepresent invention has at least two membrane module banks for the highpermeate pressure section (stage 1) and at least two membrane modulebanks for the low permeate pressure section (stage 2). The banks can beof the same or of a different size, depending on the requirements of theprocess. The integrated membrane unit has a common feed and non-permeatesection while the permeate section is split in two. The piping of theintegrated membrane system has two distinct permeate destinations thatare integrated into a common skid.

Since there are two permeate connections at different pressure levels,therefore, both permeate streams are withdrawn at opposite ends of thepermeate main collector. For this purpose, one or more automaticsegregation valves are installed in the permeate header. The valve(s)segregate the different membrane banks that make up the stage 1 andstage 2 membrane units and at the same time allow each stage to operateat its own permeate pressure level.

The invention assumes that each membrane stage has at least two bankswhich means that the smallest configuration of integrated system wouldhave at least four membrane module banks. In such smallestconfiguration, at least a single segregation valve should be installedto separate both membrane sections. For larger membrane systems, wherethe stage 1 and/or stage 2 are configured to have more than two modulebanks, additional segregation valves can be installed. Thisconfiguration adds flexibility to the system. In the most flexibleconfiguration, with N_(H) banks belonging to stage 1 and N_(L) banksbelonging to stage 2, a total of N_(H)+N_(L)−1 segregation valves can beinstalled. One of these valves would be in fully closed position whilethe others are in open position.

This configuration permits to create a high flexibility in the system tochange the flow ratio between the gas that is used as recycle gas to thePSA feed inlet (from stage 1 at high permeate pressure) and the gas thatis recycled to the PSA provide purge inlet (from stage 2 at low permeatepressure). Based on the required split of external PP or the recyclegas, the position of the control valves can be modified to change thenumber of membrane module banks belonging to either the stage 1 or stage2. One of the valves will be closed and segregates stage 1 from stage 2,while the other valves are open. The banks connected on thehigh-pressure side of the closed segregation valve make up stage 1,while the banks connected to the low-pressure side of the closedsegregation valve make up stage 2. To change the ratio of membrane feedgas flowing to stage 1 and stage 2 and thus the ratio of external PP orthe recycle gas, the individual stage-cut controllers of each stage canbe used. For larger changes, it may be required to change the assignmentof a bank of membrane modules from stage 1 to stage 2, or vice-versa.

To allow such changes without stopping the membrane system, positionersare installed on the automatic segregation valves to permit slow openingand avoid pressure shock waves between the high and low permeatepressure sides. When one bank is re-configured from one stage to theother stage, manipulation of the segregation valves at either side ofthe bank will permit the change in configuration. To change the bankfrom stage 1 (high pressure) to stage 2 (low pressure), the opensegregation valve connecting the bank to stage 1 is closed first andthen the closed segregation valve connecting to stage 2 is ramped open.To change the bank from stage 2 (low pressure) to stage 1 (highpressure), the open segregation valve connecting the bank to stage 2 isclosed first and the closed segregation valve connecting to stage 1 isramped open. The control system will keep the feed pressure andnon-permeate pressures high and control the stage-cut of both stage 1and stage 2 by modifying the pressure at their respective permeatesides.

Based on the specific properties of the 2 membrane sections (installedarea), the control system can calculate how much gas is going to eachbank from a single flow measurement and the quantity of installedmembrane area in stage 1 and stage 2 and can use that information incontrolling the stage cut from a single feed flow measurement instead of2 feed flow measurements. This is a benefit in the membrane skidconstruction not only through the cost reduction from the lower numberof instruments but also in the skid construction as less straight runpiping length will be required for the flow measurements.

However, in case both membrane sections operate at same temperature, thefeed heater that maintains the membrane operating temperature can becommon with a common temperature control loop and temperature controlvalve; in case both stages would operate at different temperature acommon feed heater can still be utilized but separate membrane feedtemperature control loops and corresponding control valves would berequired for both separation stages, or an additional (smaller)exchanger could be used for the higher of the operating temperatures. Inaddition, when the both stages are operating at different temperatures,segregation valves can also be added to the feed headers to direct feedgas of different temperatures to stage 1 and stage 2 separationsections, when both ends of the feed header are fed with feed gas atdifferent temperature.

A savings in operating costs through lowered power requirements as wellas a significant reduction in the size of the membrane units areaccomplished while maintaining at least the same or higher production ofhydrogen.

More specifically the present disclosure achieves approximately equalhydrogen and LPG recovery, by reducing the operating costs by ˜10% andreducing the capital costs by ˜7%. Operating costs are lowered due toreduced catalytic reforming unit net gas compression requirement(reduced recycle flow from bigger membrane) and reduced PSA tail gascompression (increased PSA recovery and reduced PSA feed gas flow).Lower capital costs are due to smaller total installed membrane area andreduced cost of compression equipment.

It should be appreciated and understood by those of ordinary skill inthe art that various other components such as valves, pumps, filters,coolers, etc. were not shown in the drawings as it is believed that thespecifics of same are well within the knowledge of those of ordinaryskill in the art and a description of same is not necessary forpracticing or understanding the embodiments of the present invention.

SPECIFIC EMBODIMENTS

While the following is described in conjunction with specificembodiments, it will be understood that this description is intended toillustrate and not limit the scope of the preceding description and theappended claims.

A first embodiment of the invention is a process of treating a net gasstream comprising sending the net gas stream to a compressor to producea compressed gas stream; sending the compressed gas stream to a pressureswing adsorption unit to produce a hydrogen product stream and a tailgas stream; sending the tail gas stream to a first membrane unit toproduce a first permeate stream and a first non-permeate stream; andsending a portion of the tail gas stream to a second membrane unit toproduce a second permeate stream and a second non-permeate stream. Anembodiment of the invention is one, any or all of prior embodiments inthis paragraph up through the first embodiment in this paragraph,further comprising compressing the tail gas stream prior to sending thetail gas stream to the first and second membrane units. An embodiment ofthe invention is one, any or all of prior embodiments in this paragraphup through the first embodiment in this paragraph, wherein the firstnon-permeate stream and the second non-permeate stream are sent to anabsorber unit to produce a fuel gas stream and a C₃₊ stream. Anembodiment of the invention is one, any or all of prior embodiments inthis paragraph up through the first embodiment in this paragraph,further comprising a control system to control flow of the tail gasstream to the first membrane unit and the second membrane unit. Anembodiment of the invention is one, any or all of prior embodiments inthis paragraph up through the first embodiment in this paragraph,wherein the second permeate stream from the second membrane unit isrecycled to the pressure swing adsorption unit as purge gas. Anembodiment of the invention is one, any or all of prior embodiments inthis paragraph up through the first embodiment in this paragraph,wherein the first membrane unit and the second membrane unit operate atsame temperatures. An embodiment of the invention is one, any or all ofprior embodiments in this paragraph up through the first embodiment inthis paragraph, wherein the first membrane unit and the second membraneunit operate at a different temperature. An embodiment of the inventionis one, any or all of prior embodiments in this paragraph up through thefirst embodiment in this paragraph, wherein the control system controlspermeate pressure of the first membrane unit and the second membraneunit by controlling a ratio of permeate flow to membrane feed flow forthe first membrane unit and the second membrane unit. An embodiment ofthe invention is one, any or all of prior embodiments in this paragraphup through the first embodiment in this paragraph, wherein the controlsystem measures an amount of flow of gas to each bank of membraneswithin each of the first membrane unit and the second membrane unit. Anembodiment of the invention is one, any or all of prior embodiments inthis paragraph up through the first embodiment in this paragraph,wherein the control system is combined with the control system for thepressure swing adsorption unit. An embodiment of the invention is one,any or all of prior embodiments in this paragraph up through the firstembodiment in this paragraph, wherein the control system for themembranes units is separate from a control unit for the pressure swingadsorption unit. An embodiment of the invention is one, any or all ofprior embodiments in this paragraph up through the first embodiment inthis paragraph, wherein the control system combines the measure of theamount of flow of gas into a single value to control an amount of gassent to the second membrane unit. An embodiment of the invention is one,any or all of prior embodiments in this paragraph up through the firstembodiment in this paragraph, combining the first membrane unit and thesecond membrane unit further comprises segregation valves withpositioners to permit slow opening of valves keeping away pressure shockwaves. An embodiment of the invention is one, any or all of priorembodiments in this paragraph up through the first embodiment in thisparagraph, wherein at least one bank of membrane units is at a lowerpressure than at least one bank of membrane units. An embodiment of theinvention is one, any or all of prior embodiments in this paragraph upthrough the first embodiment in this paragraph, wherein the firstmembrane unit and the second membrane unit each comprise at least twobanks. An embodiment of the invention is one, any or all of priorembodiments in this paragraph up through the first embodiment in thisparagraph, wherein the first membrane unit and the second membrane uniteach comprise a different membrane polymer. An embodiment of theinvention is one, any or all of prior embodiments in this paragraph upthrough the first embodiment in this paragraph, wherein the pressureswing adsorption unit comprises a protective adsorbent layer at ahydrogen product end of an adsorber bed to remove impurities from thesecond membrane permeate stream. An embodiment of the invention is one,any or all of prior embodiments in this paragraph up through the firstembodiment in this paragraph, further comprising at least one of sensingat least one parameter of the process and generating a signal or datafrom the sensing; generating and transmitting a signal; or generatingand transmitting data.

A second embodiment of the invention is a process of treating a net gasstream comprising sending the net gas stream to a compressor to producea compressed gas stream; sending the compressed gas stream to a pressureswing adsorption unit to produce a hydrogen product stream and a tailgas stream; sending the tail gas stream to a first membrane unit toproduce a first permeate stream and a first non-permeate stream; sendinga portion of the tail gas stream to a second membrane unit to produce asecond permeate stream and a second non-permeate stream; controlling aflow of the tail gas stream to the first membrane unit and the secondmembrane unit. An embodiment of the invention is one, any or all ofprior embodiments in this paragraph up through the second embodiment inthis paragraph, further comprising a controls system that controlspermeate pressure of the first membrane unit and the second membraneunit by controlling a ratio of permeate flow to membrane feed flow forthe first membrane unit and the second membrane unit.

Without further elaboration, it is believed that using the precedingdescription that one skilled in the art can utilize the presentinvention to its fullest extent and easily ascertain the essentialcharacteristics of this invention, without departing from the spirit andscope thereof, to make various changes and modifications of theinvention and to adapt it to various usages and conditions. Thepreceding preferred specific embodiments are, therefore, to be construedas merely illustrative, and not limiting the remainder of the disclosurein any way whatsoever, and that it is intended to cover variousmodifications and equivalent arrangements included within the scope ofthe appended claims.

In the foregoing, all temperatures are set forth in degrees Celsius and,all parts and percentages are by weight, unless otherwise indicated.

1. A process of treating a net gas stream comprising: sending the netgas stream to a compressor to produce a compressed gas stream; sendingthe compressed gas stream to a pressure swing adsorption unit to producea hydrogen product stream and a tail gas stream; sending the tail gasstream to a first membrane unit to produce a first permeate stream and afirst non-permeate stream; and sending a portion of the tail gas streamto a second membrane unit to produce a second permeate stream and asecond non-permeate stream.
 2. The process of claim 1, furthercomprising compressing the tail gas stream prior to sending the tail gasstream to the first and second membrane units.
 3. The process of claim1, wherein the first non-permeate stream and the second non-permeatestream are sent to an absorber unit to produce a fuel gas stream and aC₃₊ stream.
 4. The process of claim 1, further comprising a controlsystem to control flow of the tail gas stream to the first membrane unitand the second membrane unit.
 5. The process of claim 1, wherein thesecond permeate stream from the second membrane unit is recycled to thepressure swing adsorption unit as purge gas.
 6. The process of claim 1,wherein the first membrane unit and the second membrane unit operate atsame temperatures.
 7. The process of claim 1, wherein the first membraneunit and the second membrane unit operate at a different temperature. 8.The process of claim 4, wherein the control system controls permeatepressure of the first membrane unit and the second membrane unit bycontrolling a ratio of permeate flow to membrane feed flow for the firstmembrane unit and the second membrane unit.
 9. The process of claim 4,wherein the control system measures an amount of flow of gas to eachbank of membranes within each of the first membrane unit and the secondmembrane unit.
 10. The process of claim 4, wherein the control system iscombined with the control system for the pressure swing adsorption unit.11. The process of claim 4, wherein the control system for the membranesunits is separate from a control unit for the pressure swing adsorptionunit.
 12. The process of claim 8, wherein the control system combinesthe measure of the amount of flow of gas into a single value to controlan amount of gas sent to the second membrane unit.
 13. The process ofclaim 1, combining the first membrane unit and the second membrane unitfurther comprises segregation valves with positioners to permit slowopening of valves keeping away pressure shock waves.
 14. The process ofclaim 1, wherein at least one bank of membrane units is at a lowerpressure than at least one bank of membrane units.
 15. The process ofclaim 1, wherein the first membrane unit and the second membrane uniteach comprise at least two banks.
 16. The process of claim 1, whereinthe first membrane unit and the second membrane unit each comprise adifferent membrane polymer.
 17. The process of claim 1, wherein thepressure swing adsorption unit comprises a protective adsorbent layer ata hydrogen product end of an adsorber bed to remove impurities from thesecond membrane permeate stream.
 18. A process of treating a net gasstream comprising: sending the net gas stream to a compressor to producea compressed gas stream; sending the compressed gas stream to a pressureswing adsorption unit to produce a hydrogen product stream and a tailgas stream; sending the tail gas stream to a first membrane unit toproduce a first permeate stream and a first non-permeate stream; sendinga portion of the tail gas stream to a second membrane unit to produce asecond permeate stream and a second non-permeate stream; controlling aflow of the tail gas stream to the first membrane unit and the secondmembrane unit.
 19. The process of claim 18, further comprising acontrols system that controls permeate pressure of the first membraneunit and the second membrane unit by controlling a ratio of permeateflow to membrane feed flow for the first membrane unit and the secondmembrane unit.